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A system and method of dithering speed and/or torque for shifting a transmission of a vehicle having an engine, a reversible, variable displacement hydraulic motor/pump which can be driven by the engine, a hydraulic accumulator supplied by said motor/pump, and at least one reversible hydraulic drivi

A system and method of dithering speed and/or torque for shifting a transmission of a vehicle having an engine, a reversible, variable displacement hydraulic motor/pump which can be driven by the engine, a hydraulic accumulator supplied by said motor/pump, and at least one reversible hydraulic driving motor for propelling the vehicle supplied with fluid by the hydraulic accumulator and/or by said motor/pump operating as a pump. A transmission unit connects the engine with the variable displacement hydraulic motor/pump during a first mode of operation (city mode) and connects the engine to a vehicle drive wheel during a second mode of operation. The system utilizes stored hydraulic energy to dither the output of the driving motor in order to achieve quick and smooth shifts between city and highway mode, or between various ranges within the city mode.

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1. A method of engaging a transmission of a series hydraulic hybrid system having an input shaft for receiving power from a prime mover, an output drive shaft, at least one primary hydraulic pump/motor, and at least one secondary hydraulic pump/motor connected to the primary hydraulic pump/motor via

1. A method of engaging a transmission of a series hydraulic hybrid system having an input shaft for receiving power from a prime mover, an output drive shaft, at least one primary hydraulic pump/motor, and at least one secondary hydraulic pump/motor connected to the primary hydraulic pump/motor via a high pressure manifold and selectively connectable to the output drive shaft of the transmission, the method comprising the steps of: calculating a desired speed of the output shaft of the at least one secondary pump/motor via an electronic control;operating the at least one secondary pump/motor at a target speed via the electronic control, the target speed being the desired output shaft speed plus a desired speed dither provided by the electronic control; andengaging the output shaft of the at least one secondary pump/motor with the output drive shaft of the transmission via power transmission components when the target speed is reached within a prescribed speed error threshold;wherein the operating includes regulating the flow of fluid to and from an accumulator that stores pressurized fluid to the at least one secondary pump/motor to achieve the target speed. 2. A method as set forth in claim 1, wherein the engaging the transmission includes engaging a gear associated with the output shaft of the motor with a gear associated with the output drive shaft of the transmission. 3. A method as set forth in claim 1, further comprising the step of selecting a gear from a plurality of different ratio gears associated with the output shaft of the pump/motor, and the engaging includes engaging the selected gear with a gear associated with the output drive shaft of the transmission. 4. A method as set forth in claim 1, further comprising the step of selecting a gear ratio corresponding to a first gear set of a plurality of gear sets having different ratios associated with the output shaft of the pump/motor, and the engaging includes engaging a clutch associated with the output drive shaft of the transmission to thereby engage the output shaft of the pump/motor with the output drive shaft of the transmission with the selected gear ratio. 5. A method as set forth in claim 1, further comprising disengaging the output shaft of the pump/motor from the output drive shaft of the transmission, the disengaging including: calculating a desired torque of the output shaft of the at least one pump/motor;operating the at least one pump/motor at a target torque that is the desired torque plus a desired torque dither; anddisengaging the output shaft of the at least one pump/motor with the output shaft of the transmission when the target torque is commanded;wherein the operating includes regulating the flow of fluid to and from an accumulator to the pump/motor to achieve the target torque. 6. A method of disengaging a transmission of a series hydraulic hybrid system having an input shaft for receiving power from a prime mover, an output drive shaft, at least one primary hydraulic pump/motor, and at least one secondary hydraulic pump/motor connected to the primary hydraulic pump/motor via a high pressure manifold and selectively connectable to the output drive shaft of the transmission, the method comprising the steps of: entering a disengagement stage;calculating a desired torque of the output shaft of the at least one secondary pump/motor via a controller,operating the at least one secondary pump/motor at a target torque that is the desired torque plus a desired torque dither;disengaging the output shaft of the at least one secondary pump/motor with the output drive shaft of the transmission via power transmission components when the target torque is commanded;wherein the operating includes regulating a flow of fluid to and from an accumulator that stores pressurized fluid to the at least one secondary pump/motor to achieve the target torque, andwherein after disengagement of the output shaft of the pump/motor and the output drive shaft is confirmed, the controller exits from the disengagement stage. 7. A method as set forth in claim 6, wherein the disengaging the output shaft of the pump/motor from the output drive shaft includes disengaging a gear associated with the output shaft of the motor with the output drive shaft of the transmission. 8. A hybrid system for a vehicle comprising: a power drive unit having: an input shaft for receiving power from a prime mover;an output drive shaft;a primary hydraulic pump/motor having an output shaft;a secondary hydraulic pump/motor connected to the primary hydraulic pump/motor via a high pressure manifold and having an output shaft; andpower transmission components for selectively coupling the input shaft to at least one of the primary hydraulic pump/motor and the output drive shaft and for selectively coupling the secondary hydraulic pump/motor to the output drive shaft;an accumulator for storing pressurized fluid connected to both the primary and secondary pump/motors via the high pressure manifold; anda controller calculating a desired speed of the output shaft of the secondary pump/motor, operating the secondary pump/motor at a target speed that is the desired speed plus a desired speed dither, and engaging the output shaft of the secondary pump/motor with the output drive shaft unit when the target speed is reached within a prescribed speed error threshold;wherein the controller controls the high pressure manifold to supply fluid from the accumulator to the secondary pump/motor for operating the secondary pump/motor at the target speed. 9. A hybrid system as set forth in claim 8, wherein the power transmission components include a plurality of gear sets associated with the output shaft of the secondary pump/motor, each of the gear sets having a different gear ratio and being separately couplable to the output drive shaft of the power drive unit via a clutch. 10. A hybrid system as set forth in claim 9, wherein the clutch is operable to either engage a gear set of the plurality of gear sets to couple the output shaft of the pump/motor with the output drive shaft of the power drive unit, or to disengage the output shaft of the pump/motor from the output drive shaft. 11. A vehicle comprising a prime mover and the hybrid system as set forth in claim 8, wherein the prime mover is coupled to the input shaft of the power drive unit. 12. A vehicle as set forth in claim 11, further comprising at least one drive element coupled to the output drive shaft of the power drive unit for propelling the vehicle. 13. A vehicle as set forth in claim 12, wherein the at least one drive element includes a wheel. 14. A hybrid system for a vehicle comprising: a power drive unit having: an input shaft for receiving power from a prime mover;an output drive shaft;a primary hydraulic pump/motor having an output shaft;a secondary hydraulic pump/motor connected to the primary hydraulic pump/motor via a high pressure manifold and having an output shaft; andpower transmission components for selectively coupling the input shaft to at least one of the primary hydraulic pump/motor and the output drive shaft and for selectively coupling the secondary hydraulic pump/motor to the output drive shaft;an accumulator for storing pressurized fluid connected to both the primary and secondary pump/motors via the high pressure manifold; anda controller calculating a desired torque of the output shaft of the secondary pump/motor, operating the secondary pump/motor at a target torque that is the desired torque plus a desired torque dither, and disengaging the output shaft of the secondary pump/motor from the output drive shaft of the transmission when the target torque is commanded;wherein the controller regulates the high pressure manifold to supply fluid to and from an accumulator to the pump/motor to achieve the target torque. 15. A hybrid system as set forth in claim 14, wherein the power transmission components include a plurality of gear sets associated with the output shaft of the secondary pump/motor, each of the gear sets having a different gear ratio and being separately couplable to the output drive shaft of the power drive unit via a clutch. 16. A hybrid transmission as set forth in claim 15, wherein the clutch is operable to either engage a gear set of the plurality of gear sets to couple the output shaft of the pump/motor with the output drive shaft of the power drive unit, or to disengage the output shaft of the pump motor from the output drive shaft. 17. A vehicle comprising a prime mover and the hybrid system as set forth in claim 14, wherein the prime mover is coupled to the input shaft of the power drive unit. 18. A vehicle as set forth in claim 17, further comprising at least one drive element coupled to the output drive shaft of the power drive unit for propelling the vehicle. 19. A hybrid system as set forth in claim 8, wherein the controller is configured to calculate a desired torque of the output shaft of the secondary pump/motor, operate the secondary pump/motor at a target torque that is the desired torque plus a desired torque dither, and disengage the output shaft of the secondary pump/motor from the output drive shaft of the power drive unit when the target torque is commanded. 20. A hybrid system as set forth in claim 19, wherein the controller regulates the high pressure manifold to supply fluid to and from an accumulator to the pump/motor to achieve the target torque.